Data transmission system and method of passing role of primary station

Abstract

A data transmission system is described, comprising a number of stations connected together in a ring. One of the stations acts as primary and can send a message at any time. The other stations act as secondaries and can transmit messages only when they receive a polling signal (EOP) from the primary. The primary keeps a record of the number of messages sent by each of the secondaries and, after it has itself received a predetermined number of messages, the primary attempts to pass on the role of primary to the secondary that has transmitted the greatest number of messages.

We claim: 
     
       1. A method of operating a data transmission system comprising a plurality of stations connected together in a ring, the method comprising the steps: (a) selecting one of the stations to act as a primary station and selecting the other stations to act as secondary stations,   (b) operating the primary station to transmit messages and polling signals to the other stations via the ring,   (c) operating each of said secondary stations, on receipt of one of said polling signals, to transmit a message around the ring, and   (d) operating the primary station to keep a record of the number of messages transmitted by each of the secondary stations, to select the secondary station that has transmitted the greatest number of messages in a particular time period, and then to send a request signal to the selected secondary station inviting it to become the primary station.   
     
     
       2. A method according to claim 1 wherein the step of sending a request signal to the selected secondary station is performed after the primary station has received a predetermined number of messages over the ring. 
     
     
       3. A method according to claim 1 including the further steps of (a) operating a secondary station upon receipt of said request signal to send an acknowledgement signal back to the current primary station, and   (b) operating the primary station to continue to act as primary station until it has received said acknowledgement signal.   
     
     
       4. A method according to claim 3 including the further step of operating the primary station, in the event that it does not receive said acknowledgement signal within a predetermined time after sending the request signal, to send a further request signal. 
     
     
       5. A data transmission system comprising a plurality of stations connected together in a ring, each station including: (a) means for operating the station in primary mode in which the station sends messages and polling signals over the ring to the other stations,   (b) means for alternatively operating the station in a secondary mode in which the station responds to a polling signal received over the ring by sending a message over the ring, and   (c) means operative when the station is in primary mode, for keeping a record of the number of messages sent by each of the other stations, for selecting the one of the other stations that has sent the greatest number of messages in a particular time period, and for then sending a request signal to the selected station inviting it to become the primary station.   
     
     
       6. A system according to claim 5 wherein each station further includes means operative when the station is in the primary mode for setting a countdown, to a first value when the station enters the primary mode, means for decrementing countdown whenever the station receives a message, and means for sending said request signal when the countdown expires. 
     
     
       7. A system according to claim 5 wherein each station further includes means operative when the station is in the secondary mode, upon receipt of said request signal, for sending an acknowledgement signal over the ring and then entering the primary mode. 
     
     
       8. A system according to claim 7 wherein each station further includes means operative when the station is in the primary mode, upon receipt of said acknowledgement signal, for entering the secondary mode.